Export cxx module in CMake

[n0F4x]

The flux module is not installed.
I don't think it can be used with find_package this way.
See a possible implementation in the mp-units repo.

[tcbrindle]

You are correct, the module interface source file doesn't currently get installed. This is deliberate, because as far as I know there is no standard install location for module cpp files yet (unlike e.g. /usr/include for headers) and I don't want to randomly dump files in surprising places in a user's filesystem when they do make install.

From what I can tell, mp-units seems to install its module file into /usr/lib/miu (module interface unit? mp-units interface?) which to me counts as quite a surprising location!.

Once some sort of common module install location emerges (perhaps when Linux distros start shipping module files) I'll update Flux to use it.

Even without installation currently, Flux's module support can be used via CMake's FetchContent so hopefully it can also be used via vcpkg and Conan as well, although I haven't actually tried it.